I've been thinking about the origins of life and evolution lately, and a question popped into my head - is there a "point of no return" for evolution? For example, could a species of tree evolve in such a manner that would "turn it"[1] into a new species of photosynthetic bacteria? Could we, descendants of photosynthetic bacteria[2], "develop" a random mutation that would make us photosynthetic again? Or has our DNA changed too much since that time? Could all life on Earth "go back" to being little more than protozoa through evolution[3]?

I've thought about these questions and I was unable to conceive of a scenario in which being "simpler" would be an advantage, save for situations that would result in a mass-extinction event. What do you guys think?EDIT: Just to emphasize - I'm not looking for mass-extinction events as a solution to this.

Evolution is just as likely to reduce complexity as increase it, depending on it's environment. Bacteria lose unnecessary genes all the time and are able to reacquire them later. More genes = more time and energy needed to reproduce versus the competition, which may or may not be from the same species. Your scenario would seem to be extraordinarily unlikely if not impossible. Just too much complexity involved.

Disclaimer: I'm not a biologist. But going from multicellular life to bacteria, as mrbiscoop says, seems unlikely. For one thing, there's a well-established microbial world at this point; any population of multicellular organisms gradually moving into that niche by natural selection / mutation would face fierce competition already in place.

Evolution does cause organisms to modify or even lose structures (the legs of whales' land-dwelling ancestors, the wings of modern birds from dinosaur forelimbs, the human appendix, eyes in cave-bound species), but losing entire magnitudes of complexity is something else. I can't imagine a scenario where such an evolutionary history would occur either.

Which makes me curious: is there evidence in biology / paleontology for a significant reduction of complexity, in any known organism's evolutionary history? The history of life on Earth is long; has anything like the scenario Lucifer postulates ever been discovered?

I'll do a little digging on the web and see what I can find.

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Live a good life... If there are no gods, then you will be gone, but will have lived a noble life that will live on in the memories of your loved ones. I am not afraid.--Marcus Aurelius

I was watching a video this evening on iTunes, in which a physicist who became interested in biology and finding "laws" within that discipline noted that life evolves towards efficiency. Not deliberately, of course, but efficiencies that do evolve tend to trump less efficient changes in an organism. So we end up efficient in the long run.

Little things (bacteria, mice, flies) are less efficient than big things like humans and elephants. They wear out quicker. So Because the efficiency issue seems to follows biological "laws" that are only now being discovered, it is probably hard for an organism to work against parameters it has spent millions of years developing.

If you like interesting and obscure science stuff, here is a web page that has the video I just watched. I found it fascinating.

I don't know about going from human to bacteria, but from carnivore to photosynthetic? That's very possible. Especially as our climate shifts. It probably won't be humans that evolve (we don't adapt to our surroundings, we adapt our surroundings to us), but I could see animals that spend a lot of time in the sun going that route. Animals like alligators.

So, from what I can gather from your answers (sorry, ParkingPlaces, but I haven't had time to watch that video), evolution (without taking into account mass-extinction events) is a one-way thing - species evolve and become more complex; never the opposite. In addition, the answer to the question in my OP ("Is there an evolutionary point of no return?") is "yes". Is this correct?

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My names are many, yet I am One.-Orion, son of Fire and Light, Sol Invictus.

So, from what I can gather from your answers (sorry, ParkingPlaces, but I haven't had time to watch that video), evolution (without taking into account mass-extinction events) is a one-way thing - species evolve and become more complex; never the opposite. In addition, the answer to the question in my OP ("Is there an evolutionary point of no return?") is "yes". Is this correct?

Not 'complex', 'efficient'. Complexity by itself does not preclude a return to simplicity. But once a species starts paring away unnecessary elements, then it loses the ability to revert to a simpler form. Humans have diverged too much from other primates for us to return to a common ancestor (without extensive genetic engineering), but it is not specifically complexity that caused that divergence.

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Nullus In Verba, aka "Take nobody's word for it!" If you can't show it, then you don't know it.

I don't buy into the idea that we, as humans, are more efficient than a single celled organism of practically any variety. We are only more complex, and quite possibly less efficient.

Evolution is all about survival. If you can spend less resources to do the same thing (become more efficient) then you've got surplus resources to use on something else. In our case, it's the brain. 2% of the body's mass, uses 20% of its "resources".

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My names are many, yet I am One.-Orion, son of Fire and Light, Sol Invictus.

... at the risk of sounding facetious, there is a school of thought that posits multicellular life really exists as groups of bacterium that sorted out cooperative behaviour was advantageous.

That is: you exist because (in really, really simplistic terms), your spleen, lungs, and heart all decided that working together was a better plan than going it alone.. and specialized to make that happen.

*shrugs* But now that I'm at the limit of my evolutionary knowledge on the subject, I'm just going to watch with interest.

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"But to us, there is but one god, plus or minus one." - 1 Corinthians 8:6+/-2

I would suggest that just as time progresses and you cannot go back, everything's dna progresses and can therefore, following the properties of time, cannot go back. However, it's not a bad assumption that if it were the only way to get energy again, there may be a way for people to go back to photosynthesis, or lay eggs, etc. However, these are all things that require a 'reset' and a mass-extinction event..

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"If you find yourself reaching for the light, first realize that it has already touched your finger.""If I were your god, I would have no reason for judgement, and you have all told endless lies about me. Wait - you do already. I am not amused by your ignorance, thoughtlessness, and shallow mind."

I don't buy into the idea that we, as humans, are more efficient than a single celled organism of practically any variety. We are only more complex, and quite possibly less efficient.

Agreed. Many people make the false assumption that evolution has a goal, complexity for example.When I read jeremy0's post I think of the Star Trek episode with Captain Pike and the green exotic dancer. Clearly you have a thing for green skinned individuals, like Yoda.

That's because I'm more efficient at deriving all my energy from the sun through photosynthesis, instead of you humans converting the sun's energy into vitamin D. When 900 years old you become, look so good, you will not...

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"If you find yourself reaching for the light, first realize that it has already touched your finger.""If I were your god, I would have no reason for judgement, and you have all told endless lies about me. Wait - you do already. I am not amused by your ignorance, thoughtlessness, and shallow mind."

There are different ways to measure efficiency, I guess. Human metabolism (and the metabolism of all larger animals) is lower than that of small animals. I perhaps incorrectly interpret that as being more efficient. We don't burn energy as fast, we don't wear out cell components as fast, we don't die as quickly.

Successfully evolved traits and abilities and such are all byproducts of a process that has no goal. Things that work get to keep working as long as everything continues to go well. Things that don't work die off quickly no matter how well everything else is going.

Presumably humans could eventually simplify in the sense that we could loose our presumably useless appendix or that silly nerve that leaves our brain, goes down and loops around our heart, and then goes back up to our throat could find a new route and be shorter. But I've never heard it suggested that we could devolve into single-celled life forms.

There are different ways to measure efficiency, I guess. Human metabolism (and the metabolism of all larger animals) is lower than that of small animals. I perhaps incorrectly interpret that as being more efficient. We don't burn energy as fast, we don't wear out cell components as fast, we don't die as quickly.

But see, you're making a point that longer life = more efficient. But that's not the case. According to what I understand from this statement, you're actually trying to say that our lower efficiency may be helping to extend our lives.

That is both true and false.

Specimens of the black coral genus Leiopathes are among the oldest continuously living organisms on the planet: around 4,265 years old.1 And some species of sponges are thought to be 10,000 years old. So the benefits of complexity are obviously extremely limited. In fact, it could easily be argued that complexity contributes to a shorter life span in multi-cellular organisms.

Even if life didn't begin with such beings, they're still one of the oldest types of beings in existence.

It's not likely, because no other mammal has done it. Mostly because they have fur. We have some photosynthetic processes involving vitamin D. It would be possible for some individuals to become slightly photosynthetic, but unless it was strongly useful, genetic drift would get rid of it again.

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When you have eliminated the impossible, whatever remains, however improbable, must be bleedn obvious.

Even if life didn't begin with such beings, they're still one of the oldest types of beings in existence.

It's not likely, because no other mammal has done it. Mostly because they have fur. We have some photosynthetic processes involving vitamin D. It would be possible for some individuals to become slightly photosynthetic, but unless it was strongly useful, genetic drift would get rid of it again.

Exactly. Think of all the different vitamins and sources of different nutrients our bodies now need. The sun would have to do a hell of a lot for us to revert back to something as simple as pure photosynthesis...

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"If you find yourself reaching for the light, first realize that it has already touched your finger.""If I were your god, I would have no reason for judgement, and you have all told endless lies about me. Wait - you do already. I am not amused by your ignorance, thoughtlessness, and shallow mind."

I would suggest that just as time progresses and you cannot go back, everything's dna progresses and can therefore, following the properties of time, cannot go back.

There's not a specific 'progression of dna' that's being followed.

Genes simply replicate, and there will be genetic differences between the originals and the offspring. Sometimes these differences don't even manifest in any physically noticeable way, and sometimes they do.

However, it's not a bad assumption that if it were the only way to get energy again, there may be a way for people to go back to photosynthesis, or lay eggs, etc. However, these are all things that require a 'reset' and a mass-extinction event..

Evolution doesn't happen because a species 'needs' to find a way to survive, nor does it require a mass-extinction event.

All that happens is that when something is born, it has a different genetic structure than the organism that it came from.

I see no reason to assume that some animal can't be born with the ability to gain energy from the sun, given the required genetic, and resulting biological, structure.

I see no reason to assume that some animal can't be born with the ability to gain energy from the sun, given the required genetic, and resulting biological, structure.

I'll bet you a dollar that no form of mammal ever goes back to pure photosynthesis... Uh, we got a lot of time to wait.

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"If you find yourself reaching for the light, first realize that it has already touched your finger.""If I were your god, I would have no reason for judgement, and you have all told endless lies about me. Wait - you do already. I am not amused by your ignorance, thoughtlessness, and shallow mind."

All the energy we consume comes from the sun. Plants produce carbohydrates via photosynthesis and we eat plants. Carnivores/omnivores eat animals that eat plants. Vertebrates and most other complex animals aren't going to be developing photosynthesis as an energy supply any time soon.

Again, evolution doesn't occur because there's a 'need' for something. It happens because there's a change in the genetic structure.

If an organism survives and passes on new traits, that's what happens. It's not even necessary for the organism without the new traits to be replaced.

Sure, it's a longshot for photosynthesis or something analogous to occur in an animal's physiology, and it's even more of a longshot for this to continue to the point of an animal becoming completely dependent on photosynthesis, but it's still possible. (Even in the case of plants, they still require the uptake of nutrients, so it's not a matter of an animal becoming independent of any material intake).

I know I'm simplifying here, but here's a bit of an outline of how I see that it could happen -

Imagine a series of changes do take place where an animal produces cells that manufacture compounds that its physiology already has a use for, that previous incarnations needed to ingest in order to acquire. These cells wouldn't even necessarily be in their skin, they could be blood-born.

These changes aren't necessarily, and most likely won't be, a single generation manifestation of photosynthesis. The process may even be happening already. The precursors to photosynthetic structures would just be something the organism happens to produce, but doesn't have an immediate use for. As long as they aren't directly poisoning the organism, or wasting too much of its energy on unused structures and in turn causing the organism's demise, they would continue to manifest in future generations.

Eventually, a photosynthetic structure would come about. Or not. But if it does, the animal lives with it, benefits from it, and passes on yet another tiny tool to improve the chances of its offspring's survival.

At this point, it would only be a supplemental photosynthesis. Pure dependence (this, I don't think would happen in nature) would require a lot more to change, and even if this did happen there would still be a need for the organism to acquire raw material.

To move closer to pure dependence on photosynthesis, the animal would have to use very little energy. It takes days of photosynthesis for a plant to gather enough energy for an animal to survive on for a few minutes or hours.

The animal would also have to be less massive than its ancestors, again due to the overwhelming energy constraints.

It would also most likely be very docile, as well as exclusively diurnal.

For these reasons, sure, I don't think an animal would ever come out of nature to be completely photosynthetic, but there's still always a possibility.

I do think it's a much stronger possibility that a supplemental photosynthesis can occur in animals, however.

As was said earlier, the time needed for plants to produce enough carbohydrates for an animal to live a short time on makes it more or less impossible to have any form of autotrophic animal. I won't even get into the area needed for photosynthesis for producing any significant amount of energy compared to the animals weight.

I'm also going to go out on a limb and say that the metazoan lineage has never possessed any genes for photosynthesis. Litterature on the evolution of metazoan and plant cells mostly agree that what we know as plant cells formed when eukaryotic cells for some reason started an endosymbiotic relationship with photosynthesizing prokaryotes (now known as chloroplasts) AND heterotrophic prokaryotes (mitochondria). Animal cells on the other hand only had, and still have, an endosymbiosis with heterotrophic prokaryotes. To this day, the photosynthesis of plants is strictly bound to the chloroplasts.

So I'm afraid the answer to your question about de-evolution is that it won't get us something that we never had.

Edit: Bah, had so many interruptions when writing I sort of forgot a bit of the post that I was replying to . Anyway, trees could probably evolve towards simpler forms provided that there are some free ecological niches available. No doubt some other, already simpler autotrophic organism would get there first, but it is concievable.